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The Solar System 5 The Solar System R. Lynne Jones, Steven R. Chesley, Paul A. Abell, Michael E. Brown, Josef Durech,ˇ Yanga R. Fern´andez,Alan W. Harris, Matt J. Holman, Zeljkoˇ Ivezi´c,R. Jedicke, Mikko Kaasalainen, Nathan A. Kaib, Zoran Kneˇzevi´c,Andrea Milani, Alex Parker, Stephen T. Ridgway, David E. Trilling, Bojan Vrˇsnak LSST will provide huge advances in our knowledge of millions of astronomical objects “close to home’”– the small bodies in our Solar System. Previous studies of these small bodies have led to dramatic changes in our understanding of the process of planet formation and evolution, and the relationship between our Solar System and other systems. Beyond providing asteroid targets for space missions or igniting popular interest in observing a new comet or learning about a new distant icy dwarf planet, these small bodies also serve as large populations of “test particles,” recording the dynamical history of the giant planets, revealing the nature of the Solar System impactor population over time, and illustrating the size distributions of planetesimals, which were the building blocks of planets. In this chapter, a brief introduction to the different populations of small bodies in the Solar System (§ 5.1) is followed by a summary of the number of objects of each population that LSST is expected to find (§ 5.2). Some of the Solar System science that LSST will address is presented through the rest of the chapter, starting with the insights into planetary formation and evolution gained through the small body population orbital distributions (§ 5.3). The effects of collisional evolution in the Main Belt and Kuiper Belt are discussed in the next two sections, along with the implications for the determination of the size distribution in the Main Belt (§ 5.4) and possibilities for identifying wide binaries and understanding the environment in the early outer Solar System in § 5.5. Utilizing a “shift and stack” method for delving deeper into the faint end of the luminosity function (and thus to the smallest sizes) is discussed in § 5.6, and the likelihood of deriving physical properties of individual objects from light curves is discussed in the next section (§ 5.7). The newly evolving understanding of the overlaps between different populations (such as the relationships between Centaurs and Oort Cloud objects) and LSST’s potential contribution is discussed in the next section (§ 5.8). Investigations into the properties of comets are described in § 5.9, and using them to map the solar wind is discussed in § 5.10. The impact hazard from Near-Earth Asteroids (§ 5.11) and potential of spacecraft missions to LSST-discovered Near-Earth Asteroids (§ 5.12) concludes the chapter. 5.1 A Brief Overview of Solar System Small Body Populations Steven R. Chesley, Alan W. Harris, R. Lynne Jones 97 Chapter 5: The Solar System A quick overview of the different populations of small objects of our Solar System, which are generally divided on the basis of their current dynamics, is: • Near-Earth Asteroids (NEAs) are defined as any asteroid in an orbit that comes within 1.3 astronomical unit (AU) of the Sun (well inside the orbit of Mars). Within this group, a subset in orbits that pass within 0.05 AU of the Earth’s orbit are termed Potentially Hazardous Asteroids (PHAs). Objects in more distant orbits pose no hazard of Earth impact over the next century or so, thus it suffices for impact monitoring to pay special attention to this subset of all NEAs. Most NEAs have evolved into planet-crossing orbits from the Main Asteroid Belt, although some are believed to be extinct comets and some are still active comets. • Most of the inner Solar System small bodies are Main Belt Asteroids (MBAs), lying between the orbits of Mars and Jupiter. Much of the orbital space in this range is stable for billions of years. Thus objects larger than 200 km found there are probably primordial, left over from the formation of the Solar System. However, the zone is crossed by a number of resonances with the major planets, which can destabilize an orbit in that zone. The major resonances are clearly seen in the distribution of orbital semi-major axes in the Asteroid Belt: the resonances lead to clearing out of asteroids in such zones, called Kirkwood gaps. As the Main Belt contains most of the stable orbital space in the inner Solar System and the visual brightness of objects falls as a function of distance to the fourth power (due to reflected sunlight), the MBAs also compose the majority of observed small moving objects in the Solar System. • Trojans are asteroids in 1:1 mean-motion resonance with any planet. Jupiter has the largest group of Trojans, thus “Trojan” with no clarification generally means Jovian Trojan (“TR5” is also used below as an abbreviation for these). Jovian Trojan asteroids are found in two swarms around the L4 and L5 Lagrangian points of Jupiter’s orbit, librating around these resonance points with periods on the order of a hundred years. Their orbital eccentricity is typically smaller (<0.2) than those of Main Belt asteroids, but the inclinations are compara- ble, with a few known Trojans having inclinations larger than 30 degrees. It seems likely that each planet captured planetesimals into its Trojan resonance regions, although it is not clear at what point in the history of the Solar System this occurred or how long objects remain in Trojan orbits, as not all Trojan orbits are stable over the lifetime of the Solar System. • Beyond Neptune, the Trans-Neptunian Objects (TNOs) occupy a large area of stable orbital space. When these objects were first discovered, it was thought that they were truly primordial remnants of the solar nebula, both dynamically and chemically primordial. Further discoveries proved that this was not the case and that the TNOs have undergone significant dynamical processing over the age of the Solar System. Recent models also indicate that they are likely to have been formed much closer to the Sun than their current location, as well as being in high relative velocity, collisionally erosive orbits. Thus, they are likely to also have undergone chemical processing. TNOs can be further broken down into Scattered Disk Objects (SDOs), in orbits which are gravitationally interacting with Neptune (typically e > 0.3, q < 38 AU); Detached Objects, with perihelia beyond the gravitational perturbations of the giant planets; Resonant Objects, in mean-motion resonance (MMR) with Neptune (notably the “Plutinos,” which orbit in the 3:2 MMR like Pluto); and the Classical Kuiper Belt Objects (cKBOs), which consist of the objects with 32 < a < 48 AU on stable 98 5.2 Expected Counts for Solar System Populations orbits not strongly interacting with Neptune (see Gladman et al. 2008 for more details on classification within TNO populations). The Centaurs are dynamically similar in many ways to the SDOs, but the Centaurs cross the orbit of Neptune. • Jupiter-family comets (JFCs) are inner Solar System comets whose orbits are dominantly perturbed by Jupiter. They are presumed to have derived from the Kuiper Belt in much the same manner as the Centaur population. These objects are perturbed by the giant planets into orbits penetrating the inner Solar System and even evolve into Earth-crossing orbits. The Centaurs may be a key step in the transition from TNO to JFC. The JFCs tend to have orbital inclinations that are generally nearly ecliptic in nature. A second class of comets, so- called Long Period comets (LPCs), come from the Oort Cloud (OC) 10,000 or more AU distant, where they have been in “deep freeze” since the early formation of the planetary system. Related to this population are the Halley Family comets (HFCs), which may also originate from the Oort Cloud, but have shorter orbital periods (traditionally under 200 years). Evidence suggests that some of these HFCs may be connected to the Damocloids, a group of asteroids that have dynamical similarities to the HFCs, and may be inactive or extinct comets. A more or less constant flux of objects in the Oort Cloud is perturbed into the inner Solar System by the Galactic tide, passing stars, or other nearby massive bodies to become the LPCs and eventually HFCs. These comets are distinct from JFCs by having very nearly parabolic orbits and a nearly isotropic distribution of inclinations. Somewhat confusingly, HFCs and JFCs are both considered “short-period comets” (SPCs) despite the fact that they likely have different source regions. 5.2 Expected Counts for Solar System Populations Zeljkoˇ Ivezi´c,Steven R. Chesley, R. Lynne Jones In order to estimate expected LSST counts for populations of small solar system bodies, three sets of quantities are required: 1. the LSST sky coverage and flux sensitivity; 2. the distribution of orbital elements for each population; and 3. the absolute magnitude (size) distribution for each population. Discovery rates as a function of absolute magnitude can be computed from a known cadence and system sensitivity without knowing the actual size distribution (the relevant parameter is the difference between the limiting magnitude and absolute magnitude). For an assumed value of absolute magnitude, or a grid of magnitudes, the detection efficiency is evaluated for each modeled population. We consider only observing nights when an object was observed at least twice, and consider an object detected if there are three such pairs of detections during a single lunation. The same criterion was used in recent NASA NEA studies. Figure 5.1 summarizes our results, and Table 5.2 provides differential completeness (10%, 50%, 90%) values at various H magnitudes1. The results essentially reflect the geocentric (and for 1The absolute magnitude H of an asteroid is the apparent magnitude it would have 1 AU from both the Sun and the Earth with a phase angle of 0◦.
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